Retics should not be housed together, the only time they need to be together is for breeding.
If the female is truley ready to breed the stess level of the snakes should not be very high.
While they are young, you can use aspen or cypress mulch. As they get larger though you will probably beinclined to switch them to newspaper for cost and ease of cleaning. Every one I've got over 8-9ft is on newspaper.
Retics do not "require" a light, you might though just so you can see whats coming at you
I heat the whole room because I can dedicate a room to animals, but many people use UTHs (under tank heaters) Flexwatt heat tape, and Kaine (sp?) heat mats instead.
It is fairly easy to keep most retics in a rubbermaid type container until they get 6-8ft or so. Start them in a 28qt or so size move them up 1 or 2 more sizes and by then you'll have a 8-9ft animal and it will be ready for an adult size cage. The adult cage should be no smaller than 6' x 30" x 18". Height can vary but if your building it, thats really up to you. Animal Plastics and Vision both make very nice, reasonably priced cages to suit adult reticulated pythons.
In the wild baby retics are quite arboreal but as they get larger they spend more and more time on the ground. They are getting heavier but also they are redily able to defend themselves from pretty much anything out there. In captivity, retics do absolutly fine not having things to clinb on.
They will soak if given the opertunity but its not a requirement.
Most retics dont need a hide box, the only time I'd use a hide is for a small import that is having trouble or very skitish.
Variety is "really" nessesary for a happy retic. But the occasional Guinea pig or chicken isn't a bad thing either. Some retics get lethargic about eating and chicken or g pig will sometimes bounce them back with gusto. There is however the possibility of te snake getting hooked on only these food items but mine have never had a problem going back to their main food items. Retics are such voracious feeders that I sometimes find it hard to belive that they ever turn down food unless there is a reason.
Snakes in general do not usually need vitamin suplimentation. They are digesting whole animals so they are getting plenty of calcium, phosphurous, iron etc...and anything thats in the prey's gut benifeits them also. I don't know if there are any snakes that need D3 to make the calcium usable but I do know that retics do not require it.
-Jeff